The rearrangement catalyzed by a coenzyme B12 dependent enzyme, alpha-MG mutase, and an isomerase was studied with C14- and H3, C14-substrate. Substantial loss of tritium was observed in the conversion of alpha-methyleneglutarate to dimethylmaleate. This was probably caused by the presence of high concentration of the isomerase in the crude enzyme preparation. A cis and trans mixture of 2-amino-5-hydroxyhexanoic lactone was prepared. Tentative assignment of chemical shifts of all the proton resonance in both isomers was made with the aid of the spectra of two deuterated analogs; but some of the coupling constants were anomalous. No distinction between the cis and trans isomers could be made on the basis of these data.